In portable devices, such as smartphones and tablets, batteries provide power for lighting both the display screen and camera flash. The display screen is typically backlit using one or more White Light Emitting Diodes (WLED). The camera flash is typically a single LED. These devices may be powered, for example, by a lithium-ion battery with a usable range of 2.7V to 4.35V. Depending upon the number, quality, and operating parameters of diodes used in the display and camera flash, the battery voltage may need to be increased or decreased to drive the diodes.
A boost converter is a step-up DC-to-DC power converter that increases the voltage from the input source to the output load. A boost converter typically includes at least two switches, such as a diode and a transistor, and at least one energy storage element, such as an inductor. The transistor is used to periodically connect an energy source, such as a battery, directly to the energy storage element to store energy into the energy storage element. The energy storage element causes the converter to output a voltage higher than the input DC voltage of the energy source. The diode prevents the DC current in the output from flowing backwards. A boost converter can be used to drive multiple WLED diodes to backlight a display using a battery. When the output voltage requirement is less than the input voltage, such as when the diode voltage is less than the battery voltage, a boost converter is not desirable since the driving efficiency is poor or the voltage drop may cause an unstable condition.
A buck converter is a step-down DC-to-DC power converter that lowers the voltage from an input source, such as a battery, to an output load. To provide voltages over a range of voltages that are both higher than and lower than a battery power source, a combined buck-boost converter may be used. However, a typical buck-boost converter uses four bulky switching field-effect transistors (FETs) with extra biasing circuits for controlling the four FETs. This requires additional silicon area in the device. The buck-boost converter also introduces switching losses when integrated with a switching charger or power supply in the device.
In other solutions, two independent circuits are used to drive the back-light WLED diodes and the camera flash LED. A boost converter may be used for driving multiple WLED diodes, and a low-dropout (LDO) regulator may be used for driving a single WLED diode. When two separate circuit topologies are used, the design becomes more complex due to different PIN out connections with different digital controls to drive the different numbers of diodes and a larger PCB footprint.